Make ipn.Options.Prefs a pointer.

This is a prelude to making it truly optional, once state
management has moved into the backend. For now though, it's
still required. This change is just isolating the bubbling-up
of the pointerification into other layers.

Signed-Off-By: David Anderson <dave@natulte.net>

ipn/backend.go

@@ -53,7 +53,7 @@ type Notify struct {
 type Options struct {
 	FrontendLogID string // public logtail id used by frontend
 	ServerURL     string
-	Prefs         Prefs
+	Prefs         *Prefs
 	Notify        func(n Notify) `json:"-"`
 }